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Recent studies indicated that the atmospheric absorption of solar (short-wave,
is still very poorly understood. Uncertainties related to the atmospheric radiative transfer particularily for cloudy sky, however, are known to be major cause for uncertainties in climate models. In the consequence the apparent deficit in understanding atmospheric SW absorption
prompted a series studies that addressed the physical causes for the underpredicted atmospheric absorption.
Our research group is contributing to this research by primarily performing spectroscopic studies in the atmosphere with particular emphasis put on
(1) unkown, overlooked, or poorly characterized atmospheric absorbers, such H2O, O4, H2O-H2O, ....
(2) the measurements of atmospheric path length distrubutionof solar photons transmitted to the ground
(3) the spatial and temporal characterisation of skylight variations
In a wider sence, the group's scientific objectives thus focus on molecular spectroscopy (near UV/vis/near IR) and climate.
(a) Oxygen A-band spectroscopy:
High resolution spectroscopy of the oxygen A-band (760 Ė 780 nm) has been proven to be a powerful tool for the investigation of atmospheric radiative transfer, in particular for cloudy skies studies (for more details publications). It relies on the high resolution spectroscopy of a suite of oxygen A-band absorption lines seen in zenith scattered skylight. The photon path length information (PDF) is drawn from the varying shapes and optical thickness of individual absorption lines.
The instrumental set-up consists of a zenith viewing telescope with a narrow field of view (~1°) from which scattered skylight is directed into a Fastie-Ebert grating spectrometer (instruments: SOPRA UHRS1500 (1500 mm focal length) or Aero Laser MPP-1 (1250 mm focal length) equipped with CCD detector (Marconi EE42-10, front-illuminated, 2048 x 512 pixel). The typical FWHM spectral resolution is 13.5 pm. The high photon detection sensitivity of the CCD allows for a S/N larger than 1000 in spectra recorded in 5s.
The data evaluation relies on a nonlinear fitting of the measured atmospheric spectrum to (1) an extraterrestrial solar Fraunhofer reference spectrum, (2) modelled high resolution (0.5 pm) atmospheric oxygen A-band spectra convoluted with the instrumentís slit function and a priori given PDF shapes. The model uses spectroscopic data from HITRAN 2000, radio sonde measured atmospheric Pís and Tís on 40 atmospheric layers with a assumed Voigt type line shape.
Validation measurements of the used spectroscopic data bases are also performed using a high resolution FT-IR (Bruker IFS 120 M) in direct Sun observations.
(b) Long path absorption spectroscopy:
Long path absorption measurements aiming at a better characterization of spectroscopic characteristics of atmospheric absorbers rare usually performed by an horizontally aligned light path that is assembled over ground. The instrument is equipped with a white light source (PLI 500W xenon lamp), a telescope (diameter 30 cm), a grating spectrometer of Czerny-Turner type (600 groove/mm grating that provided a full width half maximum spectral resolution (FWHM) 0.5 nm), and a 1024 diode array detector (Hamamatsu S5931-1024).
(c) Zenith sky radiometry:
The scaling behavior of the radiance fields reflected by and transmitted through clouds has recently received considerable attention. Transition from a scale-invariant regime with one spectral exponent to another is a scale break. The characteristic scale at which the break occurs will normally have some special meaning, like radiative smoothing, scaling of cloud properties, or large-scale stationarity.
The investigations are performed with a narrow band radiomter directed to the zenith. The radiometer consists of a zenith pointing telescope containing a lens (diameter 21 mm, focal length 50 mm), an interference filter (753.2 nm, FWHM = 9.7 nm), and a Si-photodiode for light detection. A 16-bit AD-converter reads out the detector at a frequency of 2 Hz. The full viewing angle is 0.86 degree. The filter's acceptance interval (4-narrow band channels located at 380/680/750/760 ±10 nm) falls within a spectral window where absorption is very weak, between the O2 A-band (between 760 and 780 nm) and an H2O absorption band (centered at 722 nm).The observed zenith radiance time-series are analyzed with 2nd-order structure functions (SF), often referred to as SF, sometimes as the semi-variogram. SF analysis of a signal S of length N seeks scaling in statistical moments of ``increments.'' These are just absolute differences Delta S(r,n) = absolute[S(n+r)-S(n)] over scales r that can vary between 1 (sample) and an upper limit not so close to N that the sampling suffers (for more details see publications).
- Cabauw 2001: field campaign for the 4D Clouds Project at Cabauw (NL) in cooperation with the BBC campaign of CLIWA-NET
- ARM 2000: field campaign at the ARM Southern Great Plains Site, Oklahoma
- Clare 98: field campaign in Chilbolton
- LITFASS 98: field campaign in Lindenberg
- Mainz: ground based measurements
Radiative Transfer Modeling:
- Monte Carlo
- Fractal Analysis of VIS zenith sky brightness time series